This invention is directed to improvements in the process of partial oxidation of hydrocarbonaceous feedstocks to produce synthesis gas or syngas mixtures comprising H2, CO, and other gaseous materials. In particular, the improvements relate to the recovery of unreacted carbon and to temperature moderation of the gasification reactor used to produce the syngas, wherein a hydrocarbonaceous feed in admixture with a dispersion of recycled carbon-soot is introduced into a gasification system, particularly an oil-fed gasification system.
The production of syngas from hydrocarbonaceous feedstocks requires the addition of oxygen to the syngas reactor, which is also known as a syngas generator, or gasifier. Such processes also require the addition of a moderator such as CO2, steam or water to maintain the reactor temperature within permissible limits set by the economics of the operation and the limits of the reactor""s refractory materials of construction.
A problem associated with the partial oxidation gasification process is the presence of unreacted or unconverted carbon, mostly in the form of soot, in the raw syngas product. Several techniques can be used to remove the soot, the most common being to scrub the syngas product with water in a scrubber where the entrained carbon is transferred to the scrubbing water and a scrubbed soot-free syngas is produced.
The soot-containing water can be further processed using petroleum naphtha to extract the carbon from the water phase. The soot can then be extracted from the petroleum naphtha with a heavy oil. The heavy oil will then contain substantially all the entrained carbon and can be used as a feedstock for the partial oxidation reactor, thereby utilizing the energy value of the carbon. Benzene can also be used as an alternative to naphtha as an extracting agent for the carbon. After separating the carbon from the water with the benzene, the benzene is stripped off and recovered for reuse.
Another technique used to remove the unreacted or unconverted carbon is to filter the scrubbing water and to recover the carbon filter cake for further processing.
U.S. Pat. No. 3,979,188 to McCallister discloses a method of concentrating the water-carbon slurry from the reactor gaseous effluent scrubbing step to about 5 to 7 percent carbon, mixing the concentrated slurry with a fuel oil and returning the carbon/oil/water mixture without vaporization to the partial oxidation reactor as a substitute for the commonly used superheated high pressure steam moderator.
U.S. Pat. No. 4,699,631 to Marion also discloses a method for concentrating an aqueous dispersion of soot to produce a pumpable soot-water dispersion of about 0.55 to 4.0 weight percent of carbon soot, and recycling the soot-water dispersion to the gas feed to the partial oxidation gasifier.
U. K Patent No 1,321,069 to Shell International Research discloses carbon soot removal by agglomerating the soot particles with a hydrocarbon oil as an auxiliary agent. The use of an auxiliary substance that renders the soot particles hydrophobic and oleophilic is also disclosed. Once formed, the agglomerated particles are physically removed using a sieve, are dried to remove residual water, and are finally recycled back to the gasifier as a reactant.
U.S. Pat. No. 4,289,647 to Tippmer discloses a method for recovering carbon from the effluent gas of a gasification reaction. The gas is quenched with water to separate the unburned carbon and ash, and the heat of the effluent gas is used to produce steam. The wash water is then decanted to separate it into clear water, carbon-containing water, and ash mud. The carbon-containing water is used to produce steam to control the conversion of the ash oil in the gasification reaction.
Partial oxidation gasifiers usually operate at high conversion rates to minimize soot formation. High soot formation is undesirable due to unstable soot suspensions and to increased bulk viscosity. Greater soot formation means a lower cold gas efficiency unless the heat value of the soot is recovered.
The amount of water that can be used to scrub the soot is limited to that needed to function as a moderator in the partial oxidation gasifier as determined by a heat balance. The amount of soot generated is set by the carbon:oxygen ratio. Thus the amount of water cannot be varied to prevent excessive soot buildup. It is an object of this invention to separate the water and soot streams to allow for a variation of the quantity of quench water used to scrub the soot. The soot can be concentrated by recycling the quench water to the quench ring of the quench zone.
Ideally, the gasifier could be run at lower syngas conversion rates while maintaining a low viscosity in the soot/water mixture.
In the partial oxidation gasification process, the raw process gas exits from the reaction zone of the gasifier at a temperature in the range of about 1700xc2x0 F. to about 3500xc2x0 F., typically from about 2000xc2x0 F. to about 2800xc2x0 F. and at a pressure of about 200 psia to about 2500 psia, typically from about 700 psia to about 1500 psia. Particulate carbon is present in the range of nil to about 20 weight percent based on the amount of carbon in the original feed.
U.S. Pat. No. 4,021,366, to Robin et al., discloses that it is desirable to maintain the concentration of particulate carbon in the quench water in the range of nil to about 20 2 weight percent, desirably below 1.5 weight percent.
The hot raw effluent syngas stream leaving the reaction zone of the partial oxidation gasifier carries with it substantially all the particulate carbon soot produced in the reaction zone of the gasifier. The hot raw effluent syngas stream is introduced into a quench zone or chamber located below the reaction zone of the gasifier. The turbulent condition of the quench zone, caused by large volumes of syngas bubbling through the quench water present therein, helps to scrub or remove most of the soot from the syngas. The quench chamber generates quantities of soot mixed with water.
In some gasification processes, moderator in the form of steam, water or other material such as carbon dioxide is required with oil feeds. When steam is used, less oxygen is generally required and a higher cold gas efficiency is possible.
When water is used as the moderator in the gasifier, soot and water formed during the partial oxidation gasification reaction can be recycled to the gasifier, thereby minimizing or eliminating the need for soot filter cake blowdown or for a carbon extraction unit.
This invention combines the benefits of steam and water moderators by recycling soot and water into the oil feed. The water is then removed by vaporizing it at high pressure.
The simple disposal of filter cake from the filtration of quench water poses many environmental difficulties. This invention provides a novel method for utilizing the soot filter cake by recycling it to the gasifier for use as a reactant.
The present invention provides a method for the recovery of elemental carbon or soot produced during the partial oxidation reaction of a hydrocarbonaceous fuel and oxygen. An effluent stream of raw synthesis gas or syngas with entrained carbon soot is produced during the partial oxidation reaction. The effluent stream is scrubbed with water to produce a stream of soot-free syngas and a stream of water contaminated with soot. The soot is removed-from the water, generally by filtration or other equivalent means to form a filter cake. Most or all of the filter cake is recycled to the gasifier to serve as a reactant.